WalkersThey're propelled by legs which
lift to make intermittent contact with the ground

Walkers are legged robots that lift their legs in sequence
as they move. There
are a large number of walking BEAM robots around (you have
to admit, they just plain look neat!). The simplest is based
on a single motor, two legs, and a little "idler" wheel
that's drug along behind -- go here
for details.

The most common design by far is some variation or
another on a theme -- a 4-legged, 2-motor robot. These rely
on clever balance, and inter-motor timing to create a
walking gait.

Less-common, but increasingly popular, are walkers with
3, 4,
or even more motors. Obviously, the more motors a walker
has, the more varied its behavior can be (but along with
this comes an increase in both mechanical and electronic
complexity, as well as cost).

I've got more-detailed information in subsequent sections
on various designs; I've organized these sections according
to the number of motors a given walker design uses (note
that any one of these can have various numbers of legs).